[chronik.e.cash] Fix monorepo dir structure around docs

Summary:
Running into CI and npm issues with building and deployment of the docusaurus chronik site, chronik.e.cash

These issues may require additional measures like npm workspaces. However, we also have some tech debt related to my earlier implementation of the dockerfile and dir structure, which is making things more confusing.

• chronik.e.cash is a website and should be in the web folder
• there is no point in having a docs/ folder inside modules/, esp since chronik is the only site using this right now. I did this because I thought I needed the dockerfile to be named Dockerfile and be at a level high enough to get all the modules, but not so high that it conflicted with other files named Dockerfile -- this was a mistake

Test Plan:
In chronik.e.cash.Dockerfile, rewrite line 36 to be ARG NGINX_CONF=nginx-preview.conf
docker build -f chronik.e.cash.Dockerfile -t chronik-docs .
docker run --rm -p 8080:80 --name chronik-docs chronik-docs

Navigate to localhost:8080 and the docs site is there

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16717